1. Technical Field
The invention concerns the continuous in-line refining of molten aluminium and aluminium alloys.
2. Description of the Prior Art
Molten metals such as aluminium and aluminium alloys which include both small amounts of dissolved, particulate and gaseous impurities are treated “in-line” in equipment that is placed in a metal carrying launder or trough prior to casting, continuous casting and other usages.
The aluminium metal flows into the trough at the inlet, through the trough and exits at the outlet, and this occurs in a substantially continuous manner. The trough is installed typically between a heated vessel (such as a casting furnace) and a casting machine. The treatment is intended to remove: i) dissolved hydrogen, ii) solid non-metallic particulates, for example alumina and magnesia, and iii) dissolved impurities, for example Na, Li and Ca. This refining treatment has traditionally been accomplished using chlorine gas or mixtures of chlorine gas with an inert gas such as argon. This refining process is commonly referred to as “metal degassing” although it will be appreciated that it may be used for more than just degassing of the metal, since it also removes other contaminants such as ii) and iii) discussed previously.
There is environmental pressure to eliminate chlorine in such applications and although use of argon alone can accomplish some of the treatment, it is inadequate for other uses and in particular for treating magnesium-containing aluminium alloys.
The use of chloride salts has been used in some furnace based or batch rather than continuous metal treatments. In particular magnesium chloride (MgCl2), and mixtures of MgCl2 with potassium chloride (KCl) have been considered as a possible substitute for chlorine gas. However, magnesium chloride is particularly hygroscopic, and therefore inevitably contains moisture and persistently absorbs moisture from ambient air. During treatment, this moisture reacts with molten aluminium to generate hydrogen that dissolves in the molten metal, and may lead to poor quality metal.
In furnace and crucible treatments the presence of moisture in the magnesium chloride can be accepted as these are generally for non-critical applications. However, use in in-line treatments where the metal is cast immediately cast after treatment, and for critical products where hydrogen porosity is unacceptable, magnesium chloride has not been usable.
Magnesium chloride (MgCl2) has been used as a “cover flux” for in-line degassing treatment but this use compliments the use of in-line chlorine gas injection, and MgCl2 is clearly not meant as a substitute for in-line chlorine gas of injection of the molten metal.
U.S. Pat. No. 3,767,382 discloses a continuous in-line metal treatment system comprising a dispersing and separation chamber separated by baffles that allow the separation of impurities. A rotary disperser in the dispersing chamber is used to break-up the molten metal and disperse a treatment gas comprising chlorine gas and an inert gas into the metal. The cover flux disclosed includes 80% MgCl2 and moisture less than 0.1% by weight.
U.S. Pat. No. 4,138,245 discloses a means by which to remove sodium by introducing a chlorinating agent, which may be a mixture a chlorine gas and argon gas, introduced into a body of molten aluminium. Metal passes through a combination of filter-degasser bed coated with salt containing 85% MgCl2. The salt is confined to the bed and reacts to reduce sodium levels in the metal.
U.S. Pat. No. 5,772,725 discloses a method for in-line treatment of molten metal that is said to be useable with salts as well as with gaseous fluxes, without any particulars as to how this is achieved. The invention discloses a disperser/agitator adapted to disperse gases into a metal bath where the agitator rotation is inverted regularly.
U.S. Pat. No. 6,602,318 discloses a treatment vessel, such as a ladle, that uses a mixture of KCl/MgCl2 in a given weight ratio of 0.036 to remove calcium and particulates from the metal contained in the vessel. While KCl/MgCl2 is fed by way of an injection tube below the level of the molten metal near a rotating high shear dispersing impeller, thus achieving quick dispersion of the KCl/MgCl2.
EP-A-395 138 discloses a crucible treatment using various salts including salts containing up to 80% alkali metal and alkaline earth metal chlorides and including a disperser apparatus for handling such salts, which includes a co-injection of solids with an inert gas through a hollow shaft of the disperser below the level of the metal and at the level of the impeller.
EP-A-1 462 530 discloses an apparatus and method of treating molten metal in a crucible. The apparatus adds salt through a hollow shaft of a disperser. A pressurized inert gas transports the salt intermittently through the hollow shaft and into the metal in the crucible to the level of the impeller. The system may be used with a range of salt fluxes.
Therefore, all prior art either uses chlorine gas for refining the aluminium metal or is in a static crucible or in-line vessel which allows long residence times for the removal of impurities. Therefore there remains the problem of efficient in-line continuous refining of molten aluminium and aluminium alloys in troughs, without the use of chlorine gas.